Re: PGP-encrypted usenet posts (or similar)
"a unique key for each and every person you want to send important stuff to"
"Unless I'm missing something?"
You are indeed missing something.
PGP-like crypto will do what you want, but you distribute ONE key, your public key, to everyone, and they give you copies of their public keys. You use PGP to encrypt your message with the intended recipient's (or recipients') public key(s), and only that recipient / those recipients can decrypt it. You can also sign the message with your private key, and they can use their copies of your public key to know it's from you.
You then post the message on a suitable newsgroup (alt.anonymous.messages is a traditional candidate, assuming it still exists), preferably with a tag that will enable others to find it. To disconnect it from you a bit, post it via a chain of anonymous remailers.
All you need then is a way to hoover up the contents of alt.anonymous.messages (or similar) and find your messages. For traffic analysis purposes, of course, you hoover up everything so that nobody can tell which are your messages by looking at which messages you pick out.
Summary: none of this stuff is new. I last used this type of stuff around 12 years ago.